Ion exchange polymer electrolytes and their dispersion in a liquid medium are an essential part of fuel cells and other electrochemical applications. In fuel cells, electrochemical reactions occur either in acidic or alkaline media. In acidic environments, proton exchange membranes offer the required combination of adequate longevity and good conductivity at relatively low temperatures (25-100° C.). Whereas fuel cells and electrolytes employ proton exchange membranes, alkaline fuel cells require anion-conducting electrolytes. In alkaline environments, the efficiency of the oxygen reduction reaction is much higher than in acidic conditions, which allows the use of low-cost, abundant electro-catalysts as opposed to precious metal catalysts.
Traditionally, alkaline fuel cells use an aqueous solution of potassium hydroxide as the electrolyte, with typical concentrations of about 30%. A major operating constraint is the requirement for low carbon dioxide concentrations in the oxidant feed stream, as carbon dioxide can result in the formation of carbonate precipitates. One approach for addressing this issue is the use of solid anion-conducting polymer electrolytes as membranes and ionomers at the fuel cell electrode. Alkaline fuel cell systems based on such membranes and ionomers utilize the desirable properties of the solid electrolytes, such as the lack of requirement of liquid electrolyte circulation, less corrosion, and the capability of applying differential pressure and system design simplification.
A significant challenge in the area of alkaline fuel cells is the current lack of anion exchange polymer electrolytes that have i) good electrolyte stability in alkaline media, ii) high anionic conductivity, and iii) good processibility. In addition, high permeability of reactant gas and water for the ionomer at the fuel cell electrode has not been acquired from current anion exchange polymer electrolytes.
Without wishing to be limited by theory, the low stability of anion exchange polymer electrolytes is due to fast hydrolysis of polymer electrolytes under high pH conditions. The degradation process can be accelerated by electron-withdrawing molecules in the vicinity of cation functional group. The stability of anion exchange polymer electrolytes can be improved by incorporating bulky cations such as sulfonium, phosphazenium, and guanidinium instead of conventional tetraalkylammonium.
Recent research efforts to incorporate the bulky cations into polymer structure have been mostly limited to hydrocarbon-based polymers. While the improved stability of bulky cation incorporated hydrocarbon-based polymers over tetraalkylammonium functionalized polymers is probably the most desirable property, technical challenges related to conductivity, processibility and gas permeability still remained as major issues.
Fluorinated polymers in general have higher gas permeability than hydrocarbon-based polymers since fluorinated polymers have hydrophobicity and higher oxygen solubility than hydrocarbon-based polymer. In alkaline fuel cells, water is generated and consumed at the anode and cathode, respectively. Thus large amount of liquid water exist in the anode but additional water may need to be supplied in the cathode for the reaction. While high content of water at both electrodes is beneficial to decrease ohmic resistance of the cell, the reactant gas permeability for both electrodes can be significantly reduced in the presence of liquid water due to the flooding. Hydrophobicity and high oxygen solubility of fluorinated polymers increase gas permeability and improve the water removal ability from electrodes which is beneficial to the cell performance.
Fluorinated polymer electrolytes have higher conductivity than hydrocarbon-based polymer electrolytes at a given anion or cation concentration since hydrocarbon-based polymers have relatively lower density and thus greater inter-ionic distance than fluorinated polymers. In general, ionic conductivity can also be increased by increasing anion or cation concentration but incorporating high concentration of ionic functional group is often technically challenging to synthesize. Furthermore, polymer electrolytes with high ion concentration generally absorb excessive water which weakens their mechanical properties. High conductivity of polymer electrolytes is especially beneficial to alkaline membrane fuel cells since hydroxide conductivity in alkaline membrane fuel cells is significantly lower than proton conductivity in proton exchange membrane fuel cells due to the slower diffusion coefficient of the hydroxide ion and the larger size of cation group in the anion exchange polymer electrolytes, which dilutes the concentration of ion exchange site.
Solution or dispersion of anion exchange polymer electrolytes in a liquid medium is critical requirement for electrode processing. The limited solubility has been a significant inhibitor of successful application of alkaline fuel cells. Alkyl ammonium cation-based (and other cation-based) anion exchange polymer electrolytes may be synthesized by chloride substitution of a —CH2Cl moiety of the polymers. Because the cation form of the polymer electrolytes is directly synthesized via chloride substitution, the resultant cation functionalized polymer electrolytes has limited solubility.
Prior efforts to synthesize fluorinated anion exchange polymer electrolytes have not produced stable and practical polymer electrolytes. In addition, the stability of fluorinated polymer electrolytes comprising the directly-attached highly bulky cations is questionable since electron withdrawing characteristics of fluorine tend to weaken the stability of the bulk cations. There exists a need, therefore, for fluorinated anion conducting polymer electrolytes that are stable to chemical degradation at high pH, that have high anionic conductivity, and that have better processibility. Additionally, a need exists for methods of fabrication of high performance solid anion exchange membrane fuel cells which comprise the aforementioned anion conducting polymer electrolytes.